Thomas Brooks’ Improved Mortar Platforms on Morris Island

Discussing Battery Weed, I mentioned the platforms used on Morris Island and seen in the photo of the battery:

These platforms are a deviation from those described in the Army’s manuals of the day. In Note No. 11 of his journal of operations on Morris Island, Major Thomas Brooks indicated the regulation platforms failed at Fort Pulaski in 1862 and again on Morris Island in 1863. He complained, “all showed evidences of failing, and required important repairs after a few hours’ firing.” He elaborated on the construction of the failing platforms, giving me a frame of reference to start this anlysis:

These platforms were made of the deck plank, 9 feet long by 5 inches by 34 inches, furnished by the U.S. Ordnance Department, and were put down in conformity with the rules for laying these platforms, given in Heavy Artillery Tactics, pages 91 and 92, modified slightly by circumstances uncontrollable in field operations.

While I have not run across the specific manual cited, Heavy Artillery Tactics, the same platform was described and illustrated in both the Instruction for Heavy Artillery (1851) and John Gibbon’s The Artillerist’s Manual (1860). As described, the platforms were the standard siege platform (refer to the table of materials to the bottom of that post) as seen on display today at Fort Macon, North Carolina.

In addition to the standard platform, the manuals mentioned two alternates for use with the mortars. A ricochet platform was designed for low angle fire, allowing the artillerists to “bounce” a projectile into the target. The ricochet platform incorporated a “hurter” to deal with the horizontal recoil force. It was rated for charges of 3 pounds and below – in other words for short range fires and not applicable to Battery Weed (or nearby batteries). The other was a rail-platform described in the 1851 manual as such:

The rail-platform for siege mortars, composed of three sleepers and two rails for the cheeks of the mortar bed to slide on instead of the deck planks, is very strong, and easily constructed and laid.

The pieces, being notched to fit, are driven together at the battery, the rails being twenty-five and a-half inches apart from center to center for the 10-inch mortar, and twelve and three-fourths inches for the 8-inch mortar.

The Heavy Artillery manual included an illustration with both the standard platform and the rail-platform for handy comparison (standard platform on the left, rail platform on the right):

Notice there are no planks beneath the sleepers, which becomes important when looking at how Brooks solved the failure problems. Gibbon’s 1860 manual added that the Rail-platform was, “simple, strong, and well-suited to positions where timber can be easily procured.”

Notice the stakes used behind the sleepers and at the rear of the rails.

In my estimate of Brooks’ description of the problem, the issue was with respect to the surface area of the sleepers from which the force of recoil was imparted to the ground below. Simply put, the sleepers would bury themselves at uneven rates in the sand. To solve this, Brooks developed a hybrid arrangement of sorts:

I sought a mortar platform for direct firing which should be more durable than the above noted, and such that there could be used in its construction the heavy squared timber and plank captured on Morris Island. The simple one shown in Fig. 18 was made and put down for the 10-inch siege mortars in Battery Weed, and a similar one for the 8-inch mortar of Battery Reynolds, July 21, the only difference being in the width from center to center of the rails; in the 10-inch it is 28 inches, in the 8-inch 22 inches. The material was yellow pine. The 2-inch plank forming the foundation were thoroughly spiked fast to the rails. The platform is bedded with the vertical plank to the rear, and buried so that the upper surface of the rails is level with the terre-plein of the battery. To pickets were driven to stay it.

And here is the referenced figure:

He incorporated the standard platform’s planks to spread the recoil across a wider area. The rails elevated the mortar bed above the ground sufficiently to avoid obstruction and help spread the force over all the planks. He also used a plank behind the rails, somewhat akin to the hurter from the ricochet platform, to deal with the rearward force of recoil. Looking at the photograph again, we can see some of those components:

Brooks indicated this arrangement was employed from July 20, 1863, for siege batteries on Morris Island. The mortars fired hundreds of rounds, “with very slight repairs.”

Brooks went on to devise a similar platform for the 10-inch seacoast mortars intended, but not placed, in the position near the Beacon House:

For those heavier mortars, he incorporated planks, sleepers, and rails. As if perfecting the design, he added, “If the decking in this last plan had been extended over the whole platform, a mortar could be fired in any direction from it.”

Closing out his note, Brooks offered several lessons learned from the work with mortars on Morris Island. He preferred joints in the wood components fastened with spikes and screw bolts. Instead of the one size fits all platform described in the manuals, Brooks felt the size should be adjusted for larger mortars. Pickets did little to prevent the movement of the platform, and were not necessary. Comparing older 1840 models and new pattern 1861 mortars, Brooks pointed out the greater weight of the new mortars reduced the recoil force and thus damage to the platform. Lastly, he added, “The service of the mortar by the artillerist has much to do with the wear of the platforms.”

Let me offer up one more point in regard to the wartime photograph. If you look close, there are clues as to how the crew sighted these mortars for each firing. As discussed in an earlier post, initially the gunners would lay the mortar’s firing line using plummets. But once the intended line of fire was established, the gunners only needed to realign the mortar on that line. That was done with the arrangement of posts to the front and rear of the mortar.

A cord attached to the front post allowed the crew to establish a plum line to the rear post. After each firing, the crew pulled the cord taught and repositioned the mortar as needed using handspikes.